As the smooth-muscle relaxants, are known two types of drugs: those acting upon the contractile system, and those acting upon the relaxing system. As typical examples of the former type, may be mentioned various blockers with the excitatory, chemical transmitter receptor and calcium antagonists, and as those of the latter type, may be mentioned stimulants with the inhibitory, chemical transmitter receptor and nitrate drugs (nitric acid esters).
Recently, drugs which relax smooth muscles by activating the K.sup.+ channels have been reported as new smooth muscle relaxants.
The K.sup.+ channels in thick arteries (such as coronary and cerebral arteries) and in the tracheal smooth muscles are more rapidly and powerfully activated, compared with those in general excitatory tissues, thus functioning to prevent excessive excitation of these tissues (maintenance of the inside diameters of these arteries). However, once their physiological functions are damaged, these K.sup.+ channels suffer from electrical excitation similarly to general excitatory tissues, thus locally generating high contractile tension (spasm). The spasm generated in the coronary artery, the cerebral artery and bronchial smooth muscles are said to cause diseases such as angina pectoris, cerebrovascular diseases and asthma, and K.sup.+ channel activating agents are considered to be useful for the treatment and prevention of these diseases.
As compounds having K.sup.+ channel opening activity are known 4-(2-oxo-1-pyrrolidinyl)-2H-1-benzopyran-3-ol derivatives disclosed in JP-A-58-67683 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). The compounds of this invention are novel oxazinobenzazole derivatives which are different from the above compounds in structure and exhibit more powerful action of activating the K+channels.